"Arizona's governor and law enforcement officials should know that with this law, they are violating an international treaty."

Under the new law, police officers will be empowered to stop and interrogate any person whom they "reasonably suspect" might be in the United States illegally.

The law includes provisions allowing Arizona residents who believe the local police are not enforcing the law vigorously enough to sue a city or town.

As a result, police officers will be under pressure to make an arrest, even when in doubt, rather than risk a lawsuit, resulting in wrongful arrests and unfair enforcement, Human Rights Watch said.

While Jan Brewer, Arizona's governor, has required training for police officers to prevent "racial profiling" - acting on the basis of racial or ethnic characteristics - police will have little to go on other than an individual's appearance when choosing whom to stop.

"There are plenty of valid objections to Arizona's new immigration law ... it violates an international anti-racism treaty that Arizona state officials are obligated to uphold"

Alison Parker, US director at HRW

People of Latino descent, whether US citizens, legal residents, or undocumented persons, will be most at risk.

Human Rights Watch called on the Arizona legislature to revoke the "Support our Law Enforcement and Safe Neighbourhoods Act."

"The federal government should take all appropriate steps to ensure that provisions of the law that violate US treaty obligations are not enforced," the group said.

Parker said that there were plenty of valid objections to Arizona's new immigration law

"One that hasn't received much attention is that it violates an international anti-racism treaty that Arizona state officials are obligated to uphold," she said.

Mass demonstrations

Union members, activists and supporters of illegal immigrants staged rallies against the law on Friday outside a government building which houses the Immigration and Customs Enforcement (ICE) agency in New York City.

The law has become increasingly divisive, with Felipe Calderon, Mexico's president issuing a travel warning to Mexican citizens in Arizona.

More demonstrations are planned for the coming weeks against what will be the harshest anti-illegal immigration law in the US.

Calderon has said the law would damage relations between Mexico and Arizona

The law makes it illegal to hire day labourers off the street and to transport illegal immigrants.

It also requires state and local police to decide whether people are in the country illegally should they have "reasonable suspicion".

There is also concern in Arizona that the new law could have an impact on tourism to the state, and Mexican leaders have previously said that it may affect bilateral ties and trade.

The 3,200km US-Mexico border is said to be the busiest in the world. A total of about $1bn-worth of goods passes over the frontier in either direction every day and about 350 million people cross it annually.

It is believed that there are about 10.8 million people living illegally in the US, with an estimated 460,000 in Arizona.

The law is backed by Republicans in the state legislature and looks set to cause a standoff in November congressional elections in which the Democrats will be defending majorities in the senate and House of Representatives.